1. Field of the Invention
The present invention relates to a process for reducing sheeting during polymerization of alpha-olefins and more particularly to a process for reducing sheeting during polymerization of ethylene.
2. Summary of the Prior Art
As is well known to those skilled in the art, low pressure, high or low density polyethylenes can now be conventionally provided by a fluidized bed process utilizing several families of catalysts to produce a full range of low density and high density products. The appropriate selection of catalysts to be utilized depends in part upon the type of end product desired, i.e., high density, low density, extrusion grade, film grade resins and other criteria and are generally described e.g., in U.S. Pat. No. 4,532,311 issued on July 30, 1985.
In general, the above catalysts are introduced together with the polymerizable materials, into a reactor having an expanded section above a straight-sided section. Cycle gas enters the bottom of the reactor and passes upward through a gas distributor plate into a fluidized bed located in the straight-sided section of the vessel. The gas distributor plate serves to ensure proper gas distribution and to support the resin bed when gas flow is stopped.
Gas leaving the fluidized bed entrains resin particles. Most of these particles are disengaged as the gas passes through the expanded section where its velocity is reduced.
Unfortunately the utilization of certain type catalysts, referred to in said U.S. Patent as Type IV catalysts, as well as vanadium based catalysts are prone to cause sheeting (sheets) during production of polyolefins by polymerization of alpha olefins in the fluidized bed process.
In order to satisfy certain end use applications for ethylene resins, such as for film, injection molding and roto molding applications, these type catalysts, i.e., Type IV have been used. However, attempts to produce certain ethylene resins utilizing the Type IV catalysts or vanadium based catalysts supported on a porous silica substrate in certain fluid bed reactors, have not been entirely satisfactory from a practical commercial standpoint. This is primarily due to the formation of "sheets" in the reactor after a brief period of operation. The "sheets" can be characterized as constituting a fused polymeric material.
The sheets vary widely in size, but are similar in most respects. They are usually about 1/4 to 1/2 inch thick and are from about one to five feet long, with a few specimens even longer. They have a width of about 3 inches to more than 18 inches. The sheets have a core composed of fused polymer which is oriented in the long direction of the sheets and their surfaces are covered with granular resin which has fused to the core. The edges of the sheets can have a hairy appearance from strands of fused polymer.
After a relatively short period of time during polymerization, sheets begin to appear in the reactor, and these sheets plug product discharge systems forcing shutdown of the reactor.
Accordingly, it will be seen that there presently exists a need to improve the polymerization techniques necessary for the production of polyolefin products utilizing titanium based catalysts in fluidized bed reactors.
It is therefore an object of the present invention to provide a process to substantially reduce or eliminate the amount of sheeting which occurs during the low pressure fluidized bed polymerization of alpha olefins utilizing titanium based compounds as catalyst.
These and other objects will become readily apparent from the following description taken in conjunction with the accompanying drawing which generally indicates a typical gas phase fluidized bed polymerization process for producing high density and low density polyolefins modified slightly however to illustrate the present process for reducing or eliminating sheeting.